add: React Bootstrap Framework & Argon Dashboard 2

styles/argon-dashboard-2.0.4/assets/scss/argon-dashboard/bootstrap/mixins/_alert.scss

@@ -0,0 +1,15 @@
+// scss-docs-start alert-variant-mixin
+@mixin alert-variant($background, $border, $color) {
+  --#{$prefix}alert-color: #{$color};
+  --#{$prefix}alert-bg: #{$background};
+  --#{$prefix}alert-border-color: #{$border};
+
+  @if $enable-gradients {
+    background-image: var(--#{$prefix}gradient);
+  }
+
+  .alert-link {
+    color: shade-color($color, 20%);
+  }
+}
+// scss-docs-end alert-variant-mixin